Such a monitoring system has been described in my prior U.S. Pat. Nos. 3,747,010, 3,747,011, 3,747,012, 3,919,661, 3,932,774, 3,932,803 and 3,935,542. As particularly disclosed in U.S. Pat. Nos. 3,932,803 and 3,935,542, for example, the contactless motion detector may comprise an oscillator and an associated trigger amplifier whose energizing circuit includes a storage capacitor which is charged via a two-conductor supply network from a source of pulsating direct current by way of a current-limiting device, such as a constant-current unit or a simple resistor, shunted by an electronic switch such as thyristor. A control input of that switch, thus the gate electrode of the thyristor, is connected to a lead carrying the output signal of the detector, preferably after amplification of that signal in a transistor. The circuit including the storage capacitor and the associated current-limiting device constitutes a voltage-generating network supplying operating current to the oscillator and its trigger amplifier regardless of the conductive or nonconductive state of the thyristor or equivalent electronic switch. Certain improvements in the design of this network are the subject matter of my copending applications Ser. Nos. 782,400 filed 29 Mar. 1977, 786,805 filed 12 Apr. 1977, (now U.S. Pat. No. 4,100,479) and 787,496 filed 14 Apr. 1977, whose disclosures are hereby incorporated by reference in the present application.
As further noted in some of my prior patents, the active component of the detector need not be an oscillator but could be an impedance bridge, a field plate or any of a variety of electromagnetic, photoelectric or other transducers responding to a predetermined change of an external condition to be monitored. Reference in this connection may also be made to U.S. Pat. No. 3,833,850.
A system of this description may be used either to intensify or to reduce the energization of a load upon detection of the event to be monitored. With the load constituted by an electromagnetic relay, for example, the latter may serve to open or to close an external circuit. Not infrequently, the same monitoring system must be employed first with one and then with the other mode of operation.